Add proper and tested OIDC support on the server

Note that this will still not work on the client side; the followup CL for the client side is right after this one.

oauth/services/google.py

@@ -12,6 +12,9 @@ class GoogleOAuthService(OAuthLoginService):
   def __init__(self, config, key_name):
     super(GoogleOAuthService, self).__init__(config, key_name)
 
+  def login_enabled(self, config):
+    return config.get('FEATURE_GOOGLE_LOGIN', False)
+
   def service_id(self):
     return 'google'